1. Field of the Invention
This invention relates to an inner box for a cooking appliance and particularly it relates to the construction of an inner box for a cooking appliance, such as a microwave oven with improved seam joints.
2. Description of the Prior Art
Cooking appliances, such as microwave ovens, have been widely used, whether for business use or for household use, because of their various advantages including the ability to shorten the cooking time and to provide a constant heating. FIG. 1 is a perspective view schematically showing the typical construction of a conventional microwave oven. The microwave oven 1 has a body 2 and a door 3. The body 2 includes an inner box 4. In cooking appliances such as a microwave oven, such an inner box 4 is provided for an effective use of heat and for preventing the leakage of electric waves; thus, the body 2 has a doubled wall construction.
FIG. 2 is a perspective view schematically showing a conventional inner box. The inner box 4 comprises a front panel 5, a barrel section 6, and rear plates (not shown). The barrel section 6 is of cylindrical construction, including a top panel 9 and a barrel panel 10 of U-shaped cross section. The front panel 5 surrounds an opening 7 into the barrel section 6 to enable the putting in and taking out of food to be cooked. The barrel section 6 and front panel 5 are joined together at the peripheral edges of the panels 10 and 9 and at the peripheral edge 7A of the opening 7. Though not shown, the barrel section 6 and rear panel are joined together at the rear end edges 10B and 9B of the barrel and top panels 10 and 9 and the outer peripheral end edge of the rear panel. Therefore, the inner box 4 is in the form of a hexahedron, with the opening 7 located only in the front of the box.
In the front box 4 of the conventional microwave oven 1 constructed in the manner described above, the front panel 5, barrel panel 10, top panel 9 and rear panel are joined together by spot welding for lightly closing the inner box 4, to prevent the leakage of electric waves and to strengthen the structure. Spot welding, however, has the disadvantages of requiring much time and labor in forming the joints. Further, automating a spot welding operation is not easy, and hence it is impossible to increase the efficiency of the joining operation.
Further, the inner box 4, as an article of commerce, must have its outer visible portions painted for an acceptable appearance. If, therefore, the inner box 4 is constructed of a painted steel a separate painting operation may be omitted. Where spot welding is to be used, however, the portions to be joined, which are insulated by the paint, must have their paint removed and then repainted after the spot welding is completed. Thus, since the repainting operation cannot be avoided where spot welding is used, the merit of using painted steel sheets in the first place is lost. Therefore the case of the inner box 4 of the prior art, unpainted steel sheets are used and joined together, followed by a painting operation. Hence, in addition to the troublesome spot welding, it has been inevitable to perform a painting operation which is also troublesome and hence much time and labor have been consumed for producing the inner box 4.